1. Field of the Invention
This invention relates in general to a welding method and a welding system for a projection bolt.
2. Description of Related Art
A projection bolt comprises a shank on which screw threads are formed, a flange formed on one end of the shank, and a welding projection formed on the surface of the flange opposite to the shank side, wherein the welding projection is melted to be welded to a plate-shape work. The Japanese Patent No. 2509103 discloses an invention, in which a projection bolt, whose shank is inserted in a receiving hole formed on a movable electrode, is welded to a work, such as a steel plate and the like, when the movable electrode moves forward. The projection bolt is stopped first on the axis of the movable electrode, then is moved along the axis, and the shank of the projection bolt is inserted in the receiving hole.
The prior art described above provides a desired effect as a mechanism for moving a projection bolt and inserting the shank of the bolt in a receiving hole on an electrode. The insertion length for the shank, however, is not accurately controlled in the prior art. As a result, such a case may occur that an inserting rod resets at a stage at which the insertion length is insufficient, or that the projection bolt is inserted excessively enough to make a member holding the bolt interfere with the extremity of the electrode. The former case causes the bolt to come off from the receiving hole, due to the insufficient insertion of the bolt, and the latter case results in damaging the extremity of the electrode or the bolt holding member. These problems occur because the rod for bolt insertion is driven by fluid pressure supplied by an air cylinder and the like. To solve the problems, the behavior of components, such as the electrode, rod and so forth, must be set in a proper manner, particularly for dealing with a matter of how the shank of the projection bolt and the receiving hole on the electrode are allowed to take each relative position.
The prior art described above provides a desired effect as a mechanism for inserting a projection bolt in a receiving hole on an electrode. The prior art, however, lacks a particular consideration for improving the productivity of welding processes. Further, the prior art does not meet satisfactorily a demand that existing equipment is modified in the least extent to achieve the simplification of the equipment and reduction of an equipment cost. For such an operation that a projection bolt is welded to a small steel plate work, the above demand is particularly important. Besides, when the work moves forward to be positioned between two electrodes, a proper arrangement must be made so that the behavior of the work is not hampered. Meanwhile, in the prior art, the shank of the bolt cannot be inserted in the receiving hole of the electrode while the work is kept arranged between both electrodes.
According to one embodiment of the present invention, a welding method for a projection bolt is the method for welding the projection bolt having a shank, a flange, and a welding projection formed on the surface of the flange opposite to the shank side, to a plate shape work. The method comprises the following steps of: supporting the projection bolt by a holding head provided on a moving-back-and-forth feed rod in a state that the shank of the bolt is projected from the holding head; positioning the projection bolt on the axis of a movable electrode for moving back and forth by moving the feed rod; moving forward the movable electrode having a receiving hole for receiving the shank of the projection bolt and stopping the movable electrode from moving forward at the point that the shank is inserted into the receiving hole; retreating the holding head from the axis of the movable electrode for moving back and forth while the electrode is still; and, after the retreat of the holding head, moving forward the movable electrode again and welding the projection bolt to the work.
According to another embodiment of the present invention, a welding system for a projection bolt is the system for implementing the above welding method, that is, the welding system for welding the projection bolt having a shank, a flange, and a welding projection formed on the surface of the flange opposite to the shank side, to a plate shape work. The system comprises a welding machine on which a movable electrode and a fixed electrode are installed via supporting arms, respectively; a motor provided with a changing mechanism changing rotation into back-and-forth movement for moving the movable electrode; a receiving hole formed on the movable electrode for receiving the shank of the projection bolt; and a moving-back-and-forth feed rod provided with a holding head for holding the projection bolt in a state that the shank of the bolt is projected.
As the motor, an AC (Alternating Current) servomotor is typically employed, and the motor is equipped with the changing mechanism for changing rotation to back-and-forth movement. In general, the basic principle of the changing mechanism is that a screw shaft penetrates in a female screw block, which is rotated to make the screw shaft move back and forth, wherein a ball screw method is employed for reducing mechanical resistance produced upon operating the mechanism. In employing such a changing mechanism, the number of revolutions of the AC servomotor is set precisely according to a command from a controller, so that the extent of back-and-forth movement of the screw shaft can be controlled accurately.
The characteristics of the motor make it possible to determine precisely the stopping position of the movable electrode within a prescribed range. Therefore, the forward movement of the movable electrode is stopped instantly when the movable electrode moves toward the shank of the bolt in wait and the shank has come into the receiving hole in a prescribed length. While the movable electrode is still, the holding head retreats, and the movable electrode holding the projection bolt moves ahead again, pressing the welding projection to the work to be energized, then the welding operation ends.
In the above operation, the control of the mechanism for changing the rotation of the motor into back-and-forth movement and the drive control of the motor are executed properly and accurately, so that the stopping position of the movable electrode can be determined in a precise manner. This fact indicates that if the length of shank insertion in the receiving hole is less than a prescribed length, the holding head cannot be retreated. That is, the shank insertion with an insufficient length makes the shank incline abnormally in the receiving hole when the holding head retreats in the direction perpendicular to the electrode axis, making it difficult to insert the shank into the receiving hole smoothly. On the other hand, if the movable electrode is allowed to move forward to come too close to the holding head, the holding head and the movable electrode come to interfere, damaging each other.
It should be particularly noted in the present invention that the motor provides a control operation most appropriate for controlling the forward movement of the movable electrode. In other words, properly controlled output of the motor is sufficient enough to satisfy motion characteristics required for the movable electrode. If the forward movement of the movable electrode is controlled by a conventional technique, such as an air cylinder, it will be extremely difficult to temporarily stop the electrode within a required range and allow it to move forward again. In that case, the control will have to be made by a special two-stage cylinder. Also, according to the present invention, when the extent of forward movement by the movable electrode needs to be changed according to the replacement of a work or a projection bolt, changing the operation control memory of the motor allows the electrode to make necessary adjustment easily corresponding to the replacement of the work or bolt, eliminating the necessity for replacing an air cylinder as required in a conventional technique. Thus, controlling the movement of the electrode by the motor, as described here, has great advantages in terms of equipment arrangement and economy.
The feed rod is provided on a projection bolt feed apparatus, which can be mounted on part of the welding machine. Since the projection bolt feed apparatus is to be mounted on an existing welding machine as an additional component, a minor modification of the welding machine enables the installation of the feed apparatus on the welding machine. Therefore, it is also advantageous in terms of equipment arrangement and economy.
It is desirable that the projection bolt feed apparatus be arranged on the rear of the electrodes in the view across the length of the supporting arms. This arrangement prevents the projection bolt from interfering with a work when it comes into between the movable electrode and the fixed electrode. Both electrodes are arranged on the extremity of the supporting arms, respectively, so that the work is guided to a position between both the electrodes first, and is subjected to a welding operation smoothly without interfering with other members.
It is also desirable that a drive means for retreating the holding head from the axis of the movable electrode for moving back and forth be provided. When the movable electrode is moved forward and the shank of the bolt is inserted in the receiving hole in a prescribed length, the electrode stops moving at that point. Then, the holding head is retreated from the axis of the movable electrode for moving back and forth by the drive means. As described here, while the forward movement of the movable electrode is under accurate control, it is stopped and the holding head is retreated, so that the holding head is retreated after the bolt has been held surely in the receiving hole of the movable electrode. Therefore, the operation of moving forward the movable electrode and retreating the holding head is precisely carried out, which provides a highly reliable operability.
The feed rod can be inclined in two directions against the supporting arms, enabling the arrangement that the shank of bolt held by the holding head is stopped in the position where the bolt is coaxial with the receiving hole. With this arrangement, the feed rod is positioned so as to cross the supporting arms from a diagonal direction. As a result, the holding head holding the shank can make a linear motion to come and reach the position where the shank is coaxial with the fixed electrode.
According to another embodiment of the present invention, it is also provided a welding system for a projection bolt for welding the projection bolt having a shank, a flange, and a welding projection formed on the surface of the flange opposite to the shank side, to a plate shape work. The system comprises a stationary welding machine having multiple pairs of supporting arms, each supporting arm provided with a movable electrode and a fixed electrode, either of which having a receiving hole for inserting the shank therein. Projection bolt feed apparatuses are mounted on the stationary member of the stationary welding machine, and a work is held by a robot apparatus and is moved between both the electrodes.
The projection bolt feed apparatuses mounted on the stationary member of the stationary welding machine insert the shank of the projection bolt into each receiving hole of respective electrodes. In this state, the robot apparatus brings the work in between both the electrodes, and the movable electrodes operate, then the welding projection of the bolt and the work are pressed together, energized, and welded together. In this embodiment, both the electrodes and the projection bolt feed apparatuses are arranged as multiple pairs so that a multi-point bolt welding on one work becomes possible, which is effective for improving productivity. It is particularly emphasized that, by arranging the electrodes and bolt feed apparatuses as multiple pairs, welding operations are carried out at a plurality of spots on the work simultaneously or alternately, and the traveling distance of the work by the robot apparatus is shortened. Therefore, this embodiment makes it possible to weld a number of projection bolts in a brief operation time, having a great advantage for improving productivity.
The system described above can be achieved by modifying the arrangement of the welding unit of the stationary welding apparatus, replacing the single welding unit with a plurality of welding units, and arranging the projection bolt feed apparatuses according to the modification. Therefore, it becomes possible to reduce equipment cost and simplify equipment arrangement utilizing existing equipment. As described here, the stationary member of the stationary welding machine and the projection bolt feed apparatuses are combined as respective independent units, so that the welding of the projection bolt can be completed at a target spot on an incoming work while the bolt is kept fed on the electrodes. Besides, the modification required is simple, such that the projection feed apparatuses are mounted on an existing stationary welding machine. Since an existing robot apparatus is also used for this embodiment, no economic problem in terms of modification or arrangement arises.
Each projection feed apparatus can be arranged on the rear of the electrodes in the view across the length of the supporting arms. This arrangement prevents the projection feed apparatus from interfering with the work coming in between both the electrodes. Since each electrode is provided on the extremity of each supporting arm, the work is guided to the position between both the electrodes first, so that a welding operation is carried out as the work does not interferes with other members. This fact is very important for avoiding a problem of interfering in such a case that a work has a shape with complicated curves.
The projection bolt feed apparatus can be comprised of a holding head fitted on a moving-back-and-forth feed rod and a drive means for inserting the shank of a projection bolt in the receiving hole. The moving-back-and-forth feed rod moves into or retreats from between both the electrodes, allowing the projection bolt locked on the holding head to come in between both the electrodes. Particularly, the shank can be moved to the receiving hole in a precise manner by stopping the forward move of the feed rod in the position where the shank is coaxial with the receiving hole. The shank is inserted into the receiving hole by the output of the drive means, so that the bolt can be supplied in a highly reliable manner, following the forward move of the feed rod.
The drive means is fixed to the stationary member of the stationary welding machine, and has an output member on which a back-and-forth driving means for the feed rod is fixed. The feed rod is inclined in two directions against the supporting arms, so that the shank of the projection bolt held on the holding head is stopped in the position where the shank is coaxial with the receiving hole, and the shank is inserted into the receiving hole from that position. Since both the back-and-forth driving means for the feed rod and the drive means for guiding the shank to the receiving hole are arranged, moving the bolt in between both electrodes and inserting the shank into the receiving hole are performed without fail. This behavior is achieved mainly because of the fact that the back-and-forth driving means is fixed to the output member of the drive means. Further, the feed rod is inclined in two directions against the supporting arms, so that the projection bolt can be moved from the rear of the electrodes in between both electrodes. As a result, it becomes easy to arrange the projection bolt feed apparatus as a unit provided on the stationary member of the stationary welding machine.
Each pair of the movable electrodes, fixed electrodes, and projection bolt feed apparatuses can be arranged symmetrically on both sides divided by the center line of the work as multiple welding units. Arranged on both sides of the center of the work, each welding unit carries out a bolt welding on both sides of the work simultaneously or alternately. In addition, the above arrangement reduces the transfer distance of the work transferred by the robot apparatus, making it possible to welding a number of bolts in a short time.
Further, according to another embodiment of the present invention, it is also provided a welding system for a projection bolt for welding the projection bolt having a shank, a flange, and a welding projection formed on the surface of the flange opposite to the shank side, to a plate shape work. The system comprises a stationary welding machine on which a movable electrode and a fixed electrode are installed via supporting arms, respectively; a receiving hole formed on either of both the electrodes for receiving the shank of the projection bolt; a projection bolt feed apparatus mounted on the stationary member of the stationary welding machine; and a robot apparatus for holding a work and moving it in or from between both the electrodes; wherein the distance between the work and either of the electrodes is set so as to allow the projection feed apparatus to insert the shank into the receiving hole while the work is kept positioned between both the electrodes. Therefore, the bolt shank can be inserted into the receiving hole while the work is kept positioned between both the electrodes when the work is distanced from the electrode having the receiving hole, or made contact with the electrode without the receiving hole. Thus, bolts can be fed without moving a large work in every bolt feeding. In such a manner, welding efficiency is improved.
Hereinafter, the present invention is to be described in detail according to the drawings.